Crossroads GPS has assailed Democratic Sen. Mark Udall for shortchanging Colorado residents with his votes on health insurance; Patriot Majority USA has gone after the Republican Senate candidate in Arkansas, Tom Cotton, for being a supposed shill of the insurance industry and opposing a farm bill.

This is standard fare in the midterm elections, where spending could exceed $5 billion and outside groups specialize in attack ads. Trouble is that Colorado voters won't know which special interests may have financed the anti-Udall message nor will Arkansans know which vested interests may have ponied up for the attacks on Cotton.

This is dark money, which allows both sides to use a scam to set up supposed social welfare organizations that are a front for political partisans: Crossroads GPS is the so-called social welfare arm of Republican strategist Karl Rove's campaign-finance empire, and Patriot Majority USA is closely linked with Senate Majority Leader Harry Reid's political money apparatus. Already there has been $100 million of dark money spent in the 2014 congressional elections, according to the Center for Responsive Politics. By Election Day, that number will more than double.

There is a simple rationale for giving dark money rather than making upfront donations to political action committees, including the Democrats' Senate Majority PAC or Rove's American Crossroads PAC, which require disclosure. Dark spending provides the influence and access that comes with big money without the accountability or the attention from critics.

When the Republican Governors Association inadvertently disclosed the identities of secret dark money contributors, it embarrassed major companies such as Microsoft, Wal-Mart Stores and Aetna, which each gave $250,000, believing their donations were cloaked in anonymity. When he fought campaign-finance reform years ago, Kentucky Republican Sen. Mitch McConnell suggested the real antidote was disclosure.

Now he says forcing disclosure would run afoul of the First Amendment and would harass groups such as the Humane Society. Yet the Humane Society only would be affected if it got involved in electoral politics. In its two landmark campaign-finance decisions — the 1976 Buckley case, which is celebrated by reformers, and the 2010 Citizens United decision, which is lauded by reform opponents — the Supreme Court embraced disclosure.

The Buckley ruling specifically said disclosure of campaign contributions deters corruption. In Citizens United, which opened the floodgates of money and dismantled a major part of earlier reforms, eight justices nevertheless endorsed full disclosure that "allows voters to make informed choices in the political marketplace."

The top dark money practitioners are a diverse lot led by the Chamber of Commerce. In addition to the groups affiliated with Reid and Rove, they include the National Rifle Association, the League of Conservation Voters and the Kentucky Opportunity Coalition, a vehicle for secret money to support McConnell. The $100 million figure so far understates real dark money spending. It excludes any so-called issue ads usually thinly disguised campaign hits broadcast before Labor Day.

In North Carolina alone, in that period, Crossroads spent $3 million just on broadcast airtime, not including cable or production expenses according to Kantar Media, which tracks this spending. These groups get to take secret money by asserting that the majority of their efforts are for social welfare. Nobody believes that.

If Republicans win control of Congress they are likely to move to defund any IRS or SEC enforcement of crackdowns. One thing is certain, however: When it counts, the political beneficiaries of those secret donors will be made well aware of their benefactors and their needs.